An interesting aspect of solar-system engineering is that there is this
tradeoff between power and time. You can do a lot with little power if
you are willing to take a long time at it.

Moving asteroids and comets around, taking them apart, providing resources
to planetary bodies, can largely be done using solar power and in situ
fusion reactions. Low-thrust ion rockets or mass drivers take time to
build up delta V but are very efficient.

It should also be possible to exploit the chaotic dynamics of some parts
of the solar system, where very tiny changes can produce big outcomes.
Comets which pass close to the sun or Jupiter are good candidates
for this.